The present invention relates to materials for photodetection and identification of target analytes, in particular, to materials having light-emitting properties and, more particularly, to photoluminescent semiconductor materials for photodetection and identification of target analytes.
Porous silicon (PSi) has been extensively studied for a number of semiconductor applications since it was discovered in the late 1950""s. More recently, PSi has been shown to exhibit strong visible luminescence (Canham Appl Phys Lett 57:1046; 1990), suggesting promising applications in silicon-based opto-electronic devices. Other porous semiconductor materials such as gallium arsenide, for example, have also been studied to a lesser extent (Schmuki et al Appl Phys Lett 72:1039; 1998).
U.S. Pat. Nos. 5,338,415 and 5,453,624 (Sailor et al) describe a method for detection of chemicals by reversible quenching of PSi photoluminescence and a device for detection of organic solvents by PSi photoluminescence, respectively. A silicon wafer was electrochemically etched (anodization) with a 50:50 ethanol/hydrofluoric acid (HF) solution to produce a PSi wafer. When the PSi wafer was illuminated with a laser light source in the presence of an organic compound, such as tetrahydrofuran (THF), diethyl ether, methylene chloride (MeCl2), toluene, o-xylene, ethanol and methanol (MeOH), the inherent luminescent emission intensity of the PSi was significantly decreased (i.e., the photoluminescent response of the PSi was quenched). Also, Sailor observed that a toluene based solution of ferrocene (i.e., dicyclopentadienyl Fe (II)) resulted in a complete loss of luminescence.
Generally, Sailor suggests that the degree of quenching in the photoluminescent (PL) response tracks with the dipole moment of the compounds evaluated. Accordingly, these studies suggest that such an evaluation technique can help assess the differences in dipole moments between certain organic compounds. However, Sailor fails to suggest how such a method could distinguish between two or more compounds having similar dipole moments, but otherwise very different chemical compositions. For example, in U.S. Pat. No. 5,338,415, Sailor observed that MeCl2, MeOH and THF all had luminescent quenching ratios of about 0.1 or less. Consequently, there is only a small difference in the PL response curves produced by each of these compounds, which could be used to better characterize their respective chemical structures. Also, Sailor discloses a reversible wavelength (xe2x80x9cxcexxe2x80x9d) shift of about 30 nanometers (nm, 10xe2x88x929 m), from 670 nm to 630 nm, when PSi is exposed to THF. He suggests that the other organic compounds evaluated produce reversible quenching, but fails to suggest that the xcex shift is similar to the xcex shift observed for THF. However, on its face, it appears that Sailor is suggesting that the xcex shifts are substantially similar in magnitude and direction for all organic compounds evaluated. Accordingly, this 30 nm range provides a somewhat limited window for spectroscopically discriminating between unknown compounds.
Lin et al describe a biosensor based on induced wavelength shifts in the Fabry-Perot fringes in the visible light reflection spectrum of a thin flat film of PSi (Science 278:840; Oct. 31, 1997). Optically flat thin films of PSi, prepared by electrochemical etching with a 98% ethanol: 49% aqueous HF solution, are sufficiently transparent to display Fabry-Perot fringes in their optical reflection spectrum. A recognition element is immobilized on the flat PSi film. Subsequent binding of an analyte to the recognition element therefore results in a change in the refractive index of the PSi film and is detected as a corresponding shift in the interference pattern. The interference pattern is created by reflectance of white light and an interference pattern produced when multiple reflections of white light are directed toward a solution/PSi interface and a PSi/bulk silicon interface. Producing and maintaining nearly perfectly parallel planes between the air/PSi and PSi/bulk silicon interfaces is critical to producing precise and accurate interferometric spectra. Consequently, this technique is limited to applications where environmental conditions such as vibration, temperature and atmospheric gases can be precisely controlled.
Janshoff et al (J Am Chem Soc 120:12108-12116; 1998) also describe PSi for biosensor applications utilizing a shift in a Fabry-Perot fringe pattern, created by multiple reflections of illuminated white light on the air/PSi layer and PSi/bulk silicon interface, as a means for detecting molecular interactions of species in solution with immobilized ligands as receptors. Janshoff et al state that xe2x80x9cthe prerequisite for using porous silicon as an optical interferometric biosensor is to adjust the size as well as the geometrical shape of the pores by choosing the appropriate etching parametersxe2x80x9d (p. 12108). Thin films of silicon were made porous using electrochemical etching to produce pores having radii varying from 3 to 10 nm, a uniform depth, and cylindrical shape with an absolute surface area of about 0.1 to 0.15 m2 for samples etched into a 1 cm2 patch of silicon. Excessive porosity was found to be unsuitable for the biosensors of Janshoff et al.
Because interferometric techniques exploit a physical phenomenon, namely, reflectance of light by two different planes to produce an interference pattern, biosensor systems relying on shifts in the interference pattern as a means for detecting the presence of an analyte are typically very sensitive to vibration, temperature and atmospheric pressure changes. Furthermore, the reflective plates of the film or wafer, i.e. the air/PSi and the PSi/bulk silicon interfaces, must be parallel, otherwise an undesired shift in the interference pattern can occur. Typically, the reflective plates of the PSi film must be parallel to 25 xc3x85 (2.5 nm). This demands a high level of perfection in manufacture of the PSi wafer or film. Finally, for optimum performance, the irradiated light directed on the PSi film or wafer should be perpendicular to the reflective plates. Accordingly, pores which themselves are not perpendicular to the reflective plates affect the interference pattern of the PSi film or wafer and therefore adversely affect results obtained from such biosensors.
It would therefore be desirable to have a material useful for detecting target compounds, which can use the photoluminescence properties of porous semiconductor materials. Moreover, such material could be modified to provide increased sensitivity to quantitatively detecting low concentrations of predetermined target compounds.
According to one aspect of the present invention, there is provided a modified semiconductor composition comprising: (a) at least one a semiconductor material having a porous texture and (b) at least one recognition element, whereby when said composition is irradiated with at least one wavelength of electromagnetic radiation in the range of from about 100 nm to about 1000 nm, said composition produces at least one first luminescent response in the range of from about 200 nm to about 800 nm.
According to another aspect of the present invention, there is provided a method for producing a luminescent response, comprising: (a) providing a modified semiconductor composition comprising a semiconductor material having a porous texture and at least one recognition element; (b) irradiating at least said modified semiconductor composition with at least one wavelength of electromagnetic radiation in the range of from about 100 nm to about 1000 nm to produce at least one first luminescent response; and measuring at least the intensity or wavelength of said at least one first luminescent response.